Side-chain, ring and directing effects
Side-chain or ring, and directing
- A halogen can react in the ring or the side-chain.
- The conditions decide which.
- A group already on the ring directs the next one.
Side-chain vs ring
- with a halogen-carrier catalyst (AlCl₃) and no UV → substitution in the ring.
- with UV light and no catalyst → free-radical substitution in the side-chain.
Practice
A halogen reacts in the ring (not the side-chain) when there is:
The catalyst makes the electrophile for ring substitution; UV with no catalyst gives side-chain free-radical substitution.
Practice
Under UV light with no catalyst, the halogen substitutes:
UV light starts free-radical substitution in the alkyl side-chain.
Directing effects
| Group already present | Directs the new group to |
|---|---|
| $\text{–NH}_2$, $\text{–OH}$, $\text{–R}$ | positions 2 and 4 |
| $\text{–NO}_2$, $\text{–COOH}$, $\text{–COR}$ | position 3 |
Practice
An –OH group already on the ring directs the next substituent to:
–NH₂, –OH and –R are 2,4-directing; –NO₂, –COOH and –COR are 3-directing.
Practice
Match each group to where it directs the next substituent.
Activating groups (–OH, –NH₂, –R) direct to 2,4; deactivating groups (–NO₂, –COOH, –COR) direct to 3.
You've got it
Key idea
- ring substitution: catalyst (AlCl₃) + no UV; side-chain: UV + no catalyst (free-radical)
- a group already on the ring directs the next: –NH₂/–OH/–R → 2,4; –NO₂/–COOH/–COR → 3